A battery charging circuit and method

ABSTRACT

A method of charging a battery, the method comprising the steps of: providing a charging current to the battery; determining a property of the battery substantially continuously during charging; and varying a property of the charging current in dependence on the determined property of the battery.

DESCRIPTION OF INVENTION

Embodiments of the present invention relate to methods and systems foruse in the charging of batteries.

A basic battery charging system for recharging a battery (such aslithium ion battery) may conventionally apply constant or pulsedelectrical power to terminals of the rechargeable battery. Typically, acharging cycle for a lithium ion battery may include an initial constantcurrent operation during which the battery charging system is controlledto achieve a substantially constant supply current to the battery duringa first period of the charging cycle. Once the voltage across theterminals of the battery has reach a predetermined level, then aconventional battery charging system is typically controlled to achievea constant voltage or even a constant power across those terminalsduring this second period of the charging cycle (i.e. a constant voltageoperation of the charging cycle). Finally, once the battery issubstantially fully charged, then either the charging cycle isterminated or a maintenance or standby operation may be maintained withan occasional supply of electrical power to the battery to compensatefor self-discharge.

Most conventional chargers switch from the constant current to theconstant voltage or even a constant power operations of the chargingcycle with the battery between 60% and 80% of being full charged.

Without wishing to be bound by theory it is believed that ions passingfrom a positive electrode, of a battery being recharged, to a negativeelectrode of that battery tend to pile-up prior to intercalation at ortowards the boundary of the negative electrode which is towards thepositive electrode (i.e. at a solid-electrolyte interphase (SEI) layer).This pile-up is believed to slow the intercalation of the ions in thenegative electrode and creates a higher electrical resistance in thebattery.

Conventional charging also typically causes the SEI layer to grow (i.e.to thicken) which eventually, when sufficiently thick following a numberof charging cycles, will cause a reduction in the charge capacity of thebattery and eventually the failure of the battery. Conventional batterycharging systems use relatively complex electronic circuits in order toprovide the constant voltage or even a constant power and currentoperations required for the conventional charging cycles and requireadditional electronic circuits to provide power factor correction. Thesecomplex electronic circuits use numerous relatively expensivecomponents.

Accordingly, there is a desire to alleviate one or more problemsassociated with conventional battery charging cycles and systems.

As such, an aspect of the present invention provides a method forcharging a battery, the method comprising the steps of: providing acharging current to the battery; determining a property of the batterysubstantially continuously during charging; and varying a property ofthe charging current in dependence on the determined property of thebattery. By measuring battery properties substantially continuously, thecharging current supplied to the battery to be varied in substantiallyreal-time to provide a more optimal battery charging current, allowingfor a safe minimisation of the battery charging time. In general, morethan one battery property can be determined to provide information aboutthe battery state that can assist in determining the charging currentproperties required. Optionally, the charging current has an oscillatingDC waveform. Optionally, the charging current has a pulsed DC waveform.Using a time varying DC charging current such as these can allow forproperties, such as the internal impedance of the battery, to be moreeasily measured without interrupting the charging process. Forefficiency, at least a portion of the waveform has substantially theform of rectified sine wave. Optionally, at least a portion of thewaveform has substantially the form of a squared sine wave. These waveforms can provide favourable battery charging with a high power factorwhile being simple to generate. Optionally, the waveform has a minimumoccurring at a current value of less than about 0.5 amps, preferablyless than about 0.1 amps, more preferably about zero amps. By having awaveform that has a minimum of about zero amps, the effects of staticbuild and/or an internal potential up on the battery properties can bereduced. Optionally, the determined property is determined from thecurrent waveform. The current waveform is relatively straightforward tomeasure, and can be used to determine further properties of the battery.Optionally, the determined property is determined at at least thefrequency of the waveform. Determining the property at at least thefrequency of the waveform provide substantially continuous monitoring ofthe battery properties, with the properties being determined a minimumof once per waveform cycle. This allows for the charging current to beupdated frequently, or substantially continuously, in order to preventany overloading of the battery. Optionally, the determined property isdetermined during a rising portion of the waveform. For accuracy, thedetermined property is determined from a measurement starting at awaveform minimum. The waveform minimum provides a convenient start pointfor measurements, and can result in greater measured changes inproperties, and therefore a reduced fractional error. Optionally, thedetermined property is determined during a falling portion of thewaveform. Measuring the property during the falling part of the waveformin addition to the rising part of the waveform allows the property to bedetermined from a different start point, which can help to detect orreduce errors. Optionally, the waveform is provided at a frequency ofabout an integer multiple of a voltage supply frequency. Providing awaveform at an integer multiple of the mains voltage supply can resultin a higher power factor, increasing the efficiency of the charging.Optionally, the waveform is provided at a frequency of about twice amains voltage supply frequency, which can provide more efficientcharging. Optionally, the waveform is locked to the mains voltage supplyfrequency. By providing a waveform locked to the mains frequency, bywhich it is meant that the minimum of the current waveform and zeros ofthe voltage waveform occur at substantially the same time, a higherpower factor can be provided. Optionally, the determined property of thebattery comprises a battery internal impedance. The internal impedancecan be used to indicate the state of the battery. Optionally, thebattery internal impedance is determined from a measured change incurrent and a measured change in voltage at the battery. This allows theinternal impedance to be measured indirectly. Optionally, the determinedproperty of the battery comprises a cell internal impedance. Optionally,the determined property of the battery comprises a maximum cellimpedance. Where the battery comprises multiple cells, the cellimpedances can be different for each cell. The cell with the highestimpedance will determine the limits of a safe charging current.Optionally, the determined property of the battery comprises a batterytemperature. Optionally, the determined properties of the batterycomprises a cell temperature. The battery and/or cell temperature canindicate information about the state of the battery and/or cell, forexample whether the currently used charging current is safe. Optionally,the varied property comprises at: a current mean value; a currentmaximum value; a current amplitude; a duty cycle; and/or a chargingmode. Varying these properties of the current can alter the powersupplied to the battery, keeping it within safe limits while minimisingthe battery charge time. Optionally, the method further comprises thestep of measuring the time the battery has been charging, thereby todetermine a charge received by the battery. This allows the state ofcharge of the battery to be monitored. Optionally, the battery is alithium-ion battery. Lithium ion batteries are used in a wide range ofapplications. According to another aspect of the present invention,there is provided a battery charging system comprising: a currentsource, suitable for providing a charging current to a battery: and acontrol unit; wherein the control unit is configured to determine aproperty of the battery substantially continuously during charging;wherein the control unit is configured to vary a property of thecharging current in dependence on the determined property of thebattery. The control unit allows the charging current to be varied basedon the substantially continuously determined property of the battery toprovide a charging current with properties that are efficient forcharging while also being within safe limits so as not to damage thebattery. Optionally, the charging current is provided as an oscillatingDC waveform. Optionally, the charging current is provided as a pulsed DCwaveform. Using a time varying DC charging current such as these canallow for properties, such as the internal impedance of the battery, tobe more easily measured without interrupting the charging process.Optionally, at least a portion of the waveform has substantially theform of rectified sine wave. Optionally, at least a portion of thewaveform has substantially the form of squared sine wave. These waveforms can provide favourably battery charging while being simple togenerate. Optionally, the waveform has a minimum occurring at a currentvalue of less than about 0.5 amps, preferably less than about 0.1 amps,more preferably about zero amps. This can reduce the effects of staticbuild and/or an internal potential up on the battery properties. Forefficiency, the determined property is determined from the currentwaveform. The current waveform is relatively straightforward to measure,and can be used to determine further properties of the battery. Forsafety, one or more of the determined properties are determined at atleast the frequency of the waveform. Optionally, one or more of thedetermined properties are determined at at least twice the frequency ofthe waveform. Determining the property at at least the frequency of thewaveform provide substantially continuous monitoring of the batteryproperties, with the properties being determined a minimum of once perwaveform cycle. This allows for the charging current to be updatedfrequently, or substantially continuously, in order to prevent anyoverloading of the battery. Optionally, the determined property isdetermined during a rising portion of the waveform. For accuracy, thedetermined property is determined from a measurement starting at awaveform minimum. The waveform minimum provides a convenient start pointfor measurements, and can result in greater measured changes inproperties, and therefore a reduced fractional error. Optionally, thedetermined property is determined during a falling portion of thewaveform. Measuring the property during the falling part of the waveformin addition to the rising part of the waveform allows the property to bedetermined from a different start point, which can help to detect orreduce errors. For ease of generation, the waveform is provided at afrequency of an integer multiple of a voltage supply. Optionally, thewaveform is provided at a frequency of about twice the voltage supply.Optionally, the waveform is provided at a frequency locked with thevoltage supply. This provides a high power factor, increasingefficiency. Optionally, the current source comprises a flybackconverter. Use of a flyback converter to produce the current waveformcan reduce or remove the requirement for bulking capacitors to producethe charging current. Optionally, the system further comprises a voltagemeasurement means and/or a current measurement means. Determining thecurrent and voltage, or the current changes and voltage changes, canallow for properties of the battery to be determined withoutinterrupting the supply of current to the battery. Optionally, toindicate the state of the battery the one or more determined propertiesof the battery comprises a battery internal impedance. The internalimpedance can be used to indicate the state of the battery. Optionally,the battery internal impedance is calculated from a measured change incurrent and a measured change in voltage across the battery. This allowsthe internal impedance of the battery to be determined indirectly.Optionally, the determined property of the battery comprises one or morecell internal impedances. Optionally, the determined property of thebattery comprises a maximum cell impedance. Where the battery comprisesmultiple cells, the cell impedances can be different for each cell. Thecell with the highest impedance will determine the limits of a safecharging current. Optionally, the determined property of the batterycomprises a battery temperature. The battery and/or cell temperature canindicate information about the state of the battery. Optionally, thevaried property comprises at least one of: a current mean value; acurrent maximum value; a current amplitude; a duty cycle; and/or acharging mode. Varying these properties of the current can alter thepower supplied to the battery, keeping it within safe limits whileminimising the battery charge time. Optionally, the system furthercomprises a clock, wherein the clock is configured to measure the timethe battery has been charging, thereby to determine a charge received bythe battery. This allows the state of charge of the battery to bemonitored. To enable self-learning and updating, the control unit can beconfigured to store a plurality of the determined properties of thebattery in a database and/or look-up table. According to a furtheraspect of the present invention, there is provided a method formonitoring the internal impedance of a battery during charging, themethod comprising the steps of: providing a time varying current to thebattery; measuring a change in current substantially continuously;measuring a corresponding change in voltage substantially continuously;and determining the internal impedance of the battery from the change incurrent and the corresponding change in voltage. Optionally, the timevarying current is provided in the form of an oscillating DC waveform.Optionally, at least a portion of the waveform has substantially theform of squared sine wave. These wave forms can provide favourablybattery charging while being simple to generate. Optionally, foraccuracy the waveform has a minimum occurring at a current value of lessthan about 0.5 amps, preferably less than about 0.1 amps, morepreferably about zero amps. This can reduce the effects of static buildand/or an internal potential up on the measurement of the current changeand voltage change. Optionally, the impedance is determined at at leastthe frequency of the waveform. This provides a substantially continuousdetermination of the battery impedance. Optionally, the change incurrent and/or change in voltage are measured during a rising portion ofthe waveform. For accuracy, the change in current and/or change involtage are measured starting at a waveform minimum. The waveformminimum provides a convenient start point for measurements, and canresult in greater measured changes in current, and therefore a reducedfractional error. For flexibility, the change in current and/or changein voltage are measured during a falling portion of the waveform.According to a further aspect of the present invention, there isprovided a battery charger, the battery charger being adapted to providea current with an oscillating waveform, wherein the waveform minima areat or near 0 A. Optionally, the waveform minima are at less than 0.1 A,more preferably at less than 0.01 A and yet more preferably at less than0.001 A. Providing waveform minima at or near 0 A can reduce thebuild-up of static charges and/or internal potentials in the battery.Optionally, the peak-to-peak current is greater than 1 A, and preferablybetween about 10 A and about 30 A. Providing a large peak-to-peakcurrent can increase the accuracy of the impedance measurement. Foraccuracy, per oscillation cycle of the waveform at least 0.5 ms are ator near 0 A, preferably at least 1 ms more preferably at least 5 ms, andyet more preferably at least 10 ms. For accuracy, per oscillation cycleof the waveform at least 0.5% of the cycle period is at or near 0 A,preferably at least 1%, more preferably at least 5%, and yet preferablyat least 10%. Optionally, a plurality of battery charging systems areprovided, and each battery charging system is configured to receive aninput from a single phase of a multi-phase input and to generate acommon output. According to a further aspect of the present invention,there is provided a method of charging a battery, the method comprisingconverting an AC power source to a charging current to a battery,wherein the current has an oscillating waveform, and the waveform minimaare at or near 0 A. Optionally, the waveform is according to one or moreof the waveforms described above in relation to other aspects of theinvention. According to another aspect there is provided a batterycharger for providing a charging current to a battery, the batterycharger comprising a switched-mode power converter and a controlleradapted to vary the switching frequency and/or the duty cycle to providea current with a desired wave form. By varying the switching frequencyand/or the duty cycle the current provided can vary over time; this canenable providing a current with a desired wave form. Battery chargingwith a particular wave form can be beneficial for a number of reasonsincluding optimisation of current uptake by the battery, reduction ofbattery heating, evaluation of battery health, reduction of batterycharging time, and optimisation of battery lifespan. The controller maybe adapted to vary the switching frequency and/or the duty cycle afterone or more duty cycles of the switched-mode power converter. By varyingthe switching frequency and/or the duty cycle relatively frequently,smooth wave forms can be achieved. For smooth wave forms the controllermay be adapted to vary the switching frequency and/or the duty cycleafter each duty cycle of the switched-mode power converter. Thecontroller may be adapted to vary the switching frequency and/or theduty cycle in dependence on a voltage oscillation of an AC power source.This can enable efficient generation of an oscillating current waveform. Because of the dependency a high power factor can be achieved.This can avoid the necessity for power factor correction hardware, whichcan be bulky, large, and heavy. The controller may be adapted to varythe switching frequency and/or the duty cycle multiple times during eachvoltage oscillation of the AC power source, preferably at least threetimes during each voltage oscillation, preferably at least 10 times,more preferably at least 100 times during each voltage oscillation, andyet more preferably at least 800 times during each voltage oscillation.This can enable good replication of the voltage oscillation. Foraccuracy the battery charger may further comprise a voltage sensoradapted to provide an indication of a voltage of an AC power source tothe controller. For accuracy the voltage sensor may be adapted to sensethe voltage at least as often as the controller varies the switchingfrequency. The switching frequency and/or the duty cycle may be variedin dependence on a rectified (preferably full-wave rectified) voltageoscillation of the AC power source. Rectification can double theoscillation frequency and ensure the oscillation does not changepolarity, which is favourable for varying the switching frequency independence on the voltage oscillation. The desired wave form may have afrequency of about an integer multiple of a frequency of the AC powersource, preferably about twice a frequency of the AC power source. Thiscan provide particularly efficient generation of an oscillating currentwave form with a particularly high power factor. For efficiency thedesired wave form may be locked to a frequency of the AC power source.The desired wave form may be a sine wave, a full-wave rectified sinewave, a squared sine wave, or a combination thereof. These wave formscan provide favourably battery charging while being simple to generate.The switched-mode power converter may be a flyback transformer. Aflyback transformer can accommodate a suitable switching frequencyand/or duty cycle range. For effectiveness the switched-mode powerconverter may be adapted for operation in a critical conduction mode.The switching frequency and/or the duty cycle may be varied within arange of 1 to 1000 kHz, and preferably within a range of 10 to 500 kHz,and more preferably within a range of 40 to 200 kHz. This iscomparatively higher than typical mains power supply frequencies of 50or 60 Hz and rectified frequencies of 100 or 120 Hz, and so can enablesmooth generation of desired wave forms with frequencies between 25 and150 Hz. For adaptability the controller may be adapted to vary theswitching frequency and/or the duty cycle in dependence on a desiredmaximum charging current to a battery. The desired maximum chargingcurrent may be dependent on one or more of: a battery temperature; abattery voltage; a battery current; a battery voltage change; a batterycurrent change; a battery impedance; a charging time; and an accumulatedbattery charge. These can be particularly indicative of conditions orcircumstances where a relatively high or low charging current might beadvantageous. The controller may be adapted to vary the switchingfrequency and/or the duty cycle such that a minimum charging currentprovided to a battery is less than 1 A, preferably less than 0.1 A, morepreferably less than 0.01 A and yet more preferably approximately 0 A.This can reduce hysteresis effects and enable favourable batterybehaviour and charging. The controller may be adapted to vary theswitching frequency and/or the duty cycle such that a peak-to-peakcurrent provided to a battery is greater than 1 A, and preferablybetween about 10 A and about 30 A. This can enable effective charging ofbatteries such as lithium-ion batteries, batteries for hand-held deviceswithout excessively high and dangerous currents. The battery charger mayfurther comprise a smoother adapted to smooth the output current fromthe power converter. Each of the plurality of battery chargers may beconfigured to be connected to a multi-phase input and may be connectedto a common output. This can enable generation of a smooth wave formfrom a rectified wave form. A smooth wave form is favourable for batterycharging as it can enable effective charging. According to anotheraspect there is provided a method of charging a battery, comprising:converting an AC power source to a charging current to a battery with aswitched-mode power converter; wherein the switching frequency and/orthe duty cycle is varied in dependence on a desired wave form. Byvarying the switching frequency and/or the duty cycle the currentprovided can vary over time; this can enable providing a current with adesired wave form. Battery charging with a particular wave form can bebeneficial for a number of reasons including optimisation of currentuptake by the battery, reduction of battery heating, evaluation ofbattery health, reduction of battery charging time, and optimisation ofbattery lifespan. The switching frequency and/or the duty cycle may bevaried after one or more duty cycles of the switched-mode powerconverter, preferably after each duty cycle. By varying the switchingfrequency relatively frequently, smooth wave forms can be achieved. Theswitching frequency and/or the duty cycle may be varied in dependence ona voltage oscillation of the AC power source. This can enable efficientgeneration of an oscillating current wave form. Because of thedependency a high power factor can be achieved. This can avoid thenecessity for power factor correction hardware, which can be bulky,large, and heavy. The switching frequency and/or the duty cycle may bevaried multiple times during each voltage oscillation of the AC powersource, preferably at least three times during each voltage oscillation,preferably at least 10 times, more preferably at least 100 times, andyet more preferably at least 800 times during each voltage oscillation.This can enable good replication of the voltage oscillation. Foraccuracy the method may further comprise sensing a voltage of an ACpower source and varying the switching frequency and/or the duty cyclein dependence on the voltage, and preferably comprising sensing thevoltage at least as often as the switching frequency is varied. Theswitching frequency and/or the duty cycle may be varied within a rangeof 1 to 1000 kHz, and preferably within a range of 10 to 500 kHz, andmore preferably within a range of 40 to 200 kHz. This is comparativelyhigher than typical mains power supply frequencies of 50 or 60 Hz andrectified frequencies of 100 or 120 Hz, and so can enable smoothgeneration of desired wave forms with frequencies between 25 and 150 Hz.The switching frequency and/or the duty cycle may be varied independence on a rectified (preferably full-wave rectified) voltageoscillation of the AC power source. Rectification can double theoscillation frequency and ensure the oscillation does not changepolarity, which is favourable for varying the switching frequency independence on the voltage oscillation. For adaptability the switchingfrequency and/or the duty cycle may be varied in dependence on a desiredmaximum charging current to a battery, optionally wherein the desiredmaximum charging current is dependent on one or more of: a batterytemperature; a battery voltage; a battery current; a battery voltagechange; a battery current change; a battery impedance; a charging time;and an accumulated battery charge. These can be particularly indicativeof conditions or circumstances where a relatively high or low chargingcurrent might be advantageous. The switching frequency and/or the dutycycle may be varied such that a minimum charging current provided to abattery is less than 1 A, preferably less than 0.1 A, more preferablyless than 0.01 A and yet more preferably approximately 0 A. This canreduce hysteresis effects and enable favourable battery behaviour andcharging. The switching frequency may be varied such that a peak-to-peakcurrent provided to a battery is greater than 1 A, and preferablybetween about 10 A and about 30 A. According to another aspect there isprovided a battery charging system comprising one or more of thefollowing features:

-   -   a transformer, preferably including a first primary winding        and/or a first secondary winding;    -   a primary switch device, the primary switch device preferably        being configured to control electrical current delivered to a        transformer, for example to a first primary winding, from an        electrical power supply;    -   terminals configured to receive electrical power from a        transformer, for example from a first secondary winding of a        transformer, and configured to be connected to a battery to be        charged;    -   a control sub-system configured to control the operation of a        primary switch device;    -   a control sub-system configured to control a primary switch        device such that a transformer provides electrical power to        terminals with an electrical current waveform which has a        peak-to-peak current of greater than about 1 A;    -   a control sub-system configured to determine the electrical        impedance of a battery to be charged using an electrical current        waveform;    -   a control sub-system configured to control a primary switch        device based at least in part on a determined electrical        impedance of a battery to be charged;    -   an electrical current waveform having a frequency of greater        than about 60 Hz, preferably about 100 Hz to 120 Hz;    -   an electrical current waveform having a peak-to-peak current of        about 10 A to about 30 A;    -   an electrical current waveform varying between a low current of        about 0 A and a peak current;    -   an electrical current waveform having a frequency which is        substantially twice a frequency of the electrical power supply;    -   a primary switch device operated according to a frequency and/or        duty cycle which is varied based at least in part on an        determined electrical impedance of the battery to be charged;    -   an envelope of one or more pulses of electrical current        delivered to a transformer substantially follows a voltage        waveform of the electrical power supply;    -   a transformer is a flyback transformer;    -   an electrical impedance of a battery to be charged is determined        at a frequency of greater than about 60 Hz, and preferably at a        frequency of between about 100 Hz and about 120 Hz;    -   a transformer being connectable to a rectified AC electric power        supply;    -   a control sub-system configured to control a primary switch        device such that a transformer provides electrical power to        terminals with an electrical current waveform which has a        substantially rectified sine waveform current and/or a constant        root mean square sine waveform;    -   a control sub-system configured to control a primary switch        device such that a transformer provides electrical power to        terminals with an electrical current waveform which has a        frequency which is substantially twice a frequency of a voltage        of an AC electric power supply.    -   a control sub-system configured to control a primary switch        device to actuate the primary switch device between on- and        off-states at frequencies which vary over time; and    -   a control sub-system configured to control a primary switch        device to actuate the primary switch device between on- and        off-states at frequencies which are dependent on a magnitude of        a voltage of an AC electric power supply and/or a magnitude of a        battery current and/or on a magnitude of a battery voltage.

According to another aspect there is provided a method of operating abattery charging system comprising one or more of the followingfeatures:

-   -   the battery charging system includes a transformer, preferably        including a first primary winding, a first secondary winding,    -   the battery charging system includes a primary switch device,        the primary switch device preferably being configured to control        electrical current delivered to a first primary winding from an        electrical power supply;    -   terminals configured to receive electrical power from a first        secondary winding of a transformer and configured to be        connected to a battery to be charged;    -   a control sub-system configured to control the operation of the        primary switch device:    -   controlling a primary switch device such that a transformer        provides electrical power to terminals with an electrical        current waveform which has a peak-to-peak current of greater        than about 1 A;    -   determining an electrical impedance of a battery to be charged        using an electrical current waveform;    -   controlling a primary switch device further based at least in        part on a determined electrical impedance of a battery to be        charged;    -   controlling a primary switch device includes controlling a        primary switch device such that an electrical current waveform        has a frequency of greater than about 60 Hz, and preferably        about 100 Hz to 120 Hz;    -   controlling a primary switch device includes controlling a        primary switch device such that an electrical current waveform        has a peak-to-peak current of about 10 A to about 30 A;    -   controlling a primary switch device includes controlling a        primary switch device such that an electrical current waveform        varies between a low current of about 0 A and a peak current;    -   controlling a primary switch device includes controlling a        primary switch device such that an electrical current waveform        has a frequency which is substantially twice a frequency of an        electrical power supply;    -   controlling a primary switch device further includes controlling        the primary switch device such that a frequency and/or duty        cycle of the primary switch device is varied based at least in        part on a determined electrical impedance of a battery to be        charged;    -   controlling the primary switch device includes controlling the        primary switch device such that an envelope of one or more        pulses of electrical current delivered to a transformer        substantially follows a voltage waveform of an electrical power        supply; and    -   an electrical impedance of a battery to be charged is determined        at a frequency of greater than about 60 Hz, and preferably at a        frequency of between about 100 Hz and about 120 Hz.

As used herein, DC preferably refers to a voltage and/or a current witha constant polarity. A DC voltage and/or current may vary in time, forexample in a pulsating, oscillating, or otherwise varying waveform. In aswitched-mode power converter, variation of the switching frequencyresults in variation of the duty cycle, and vice versa. Variation of theswitching frequency can be achieved by varying the duty cycle, and viceversa. Where variation of the switching frequency is referred to herein,it may be substituted by variation of the duty cycle. The inventionextends to a battery charger and/or battery charging systemsubstantially as herein described and/or as illustrated with referenceto the figures. The invention also extends to a method for charging abattery substantially as herein described and/or as illustrated withreference to the figures. The invention also extends to a method fordetermining the impedance of a battery substantially as herein describedand/or as illustrated with reference to the figures.

Any apparatus feature as described herein may also be provided as amethod feature, and vice versa. As used herein, means plus functionfeatures may be expressed alternatively in terms of their correspondingstructure.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. In particular,method aspects may be applied to apparatus aspects, and vice versa.Furthermore, any, some and/or all features in one aspect can be appliedto any, some and/or all features in any other aspect, in any appropriatecombination.

It should also be appreciated that particular combinations of thevarious features described and defined in any aspects of the inventioncan be implemented and/or supplied and/or used independently.

Embodiments of the present invention are described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIGS. 1a and 1b schematically show what is believed to be occurringduring conventional charging of a battery;

FIG. 2 shows an embodiment of the present invention;

FIGS. 3a-3f show graphical representations of currents and voltagesduring operation of embodiments;

FIGS. 4-7 shows a battery charging system according to some embodimentsand parts thereof;

FIGS. 8-10 show the current through a first primary winding of a flybacktransformer during a charging cycle according to some embodiments; and

FIGS. 11a-11c show graphical representations of the electrical currentdelivered to a battery, the electrical voltage across the batteryterminals, and an extracted ripple voltage signal.

FIG. 12 shows a step-by-step procedure for charging a battery.

FIG. 13 shows a schematic example of a single cell battery control unitin use.

FIG. 14 shows a schematic example of a multi-cell battery control unitin use.

FIG. 15 shows a schematic example of a three phase input to a batterycharger of some embodiments.

The term “battery” as used herein is to be interpreted as referring toone or more cells capable of storing electrical charge and capable ofbeing recharged following discharge of that electrical charge. If thebattery includes multiple cells, then those cells may be connectedtogether in a circuit to form the battery. The circuit may be a seriescircuit (in which the cells are connected together in series), or aparallel circuit (in which the cells are connected together inparallel), or a series-parallel circuit (in which cells are connectedtogether in series with multiple groups of series connected cellsconnected in parallel with each other), or any combination of suchcircuits.

The terms “battery terminals”, “battery terminal”, “terminal of abattery”, and “terminals of a battery” as used herein are to beinterpreted as referring to electrical terminal or terminals of thebattery through which electrical power may be delivered by the batteryto one or more circuits and/or through which electrical power may beprovided to the battery in order to recharge the battery.

The terms “anode” and “negative electrode” are used herein substantiallyinterchangeably and are to be interpreted as references to the internalparts of a cell which form the anode of that cell. Similarly, the terms“cathode” and “positive electrode” are used herein substantiallyinterchangeably and are to be interpreted as references to the internalparts of a cell which form the cathode of that cell.

Embodiments of the invention will be described with reference to a“battery” but it will be appreciated that this is typically a referencethe operation of and processes in a single cell, and that a batterycomprising multiple cells may have a corresponding plurality of anodesand cathodes and similar operations and processes may be occurring ineach cell of the battery.

A battery (and, therefore, a cell) as described herein may be a lithiumion battery (or cell) but it is envisaged that embodiments of thepresent invention may be used with other forms of battery (and cell)too.

With reference to FIGS. 1a and 1b , and again without wishing to bebound by theory, these figures schematically show what is believed to beoccurring in a battery during a conventional charging cycle using aconstant current operation.

As can be seen from FIGS. 1a and 1b , a battery 1 includes a cell 11 andthat cell 11 includes an anode 111 and a cathode 112 in an electrolyte115. A separator 113 may positioned between the anode 111 and cathode112. The anode 111 may be connected in electrical communication with anegative terminal 12 of the battery 1 and the cathode 112 may beconnected in electrical communication with a positive terminal 13 of thebattery 1.

A solid-electrolyte interphase (SEI) layer 114 is located at or towardsthe boundary of the anode 111.

During charging of the battery 1, an electrical voltage is appliedacross the terminals 12,13 of the battery 1 to cause ions 3 to pass fromthe cathode 112, e.g. through the separator 113 and electrolyte 115, tothe anode 111 and through the SEI layer 114 to intercalate in the anode111 (during discharge, of course, the flow of ions 3 is in the oppositedirection within the battery 1).

It is believed that the ions 3 cannot intercalate in the anode 111 asquickly as they are supplied to the SEI layer 114 under a conventionalcharging cycle during a constant current operation. Therefore, a pile-upof ions 3 occurs at or towards the boundary of the anode 111 (i.e. inthe region of the SEI layer 114). This is schematically represented inFIG. 1 b.

As explained herein, it is believed that this—in turn—increases theelectrical resistance within the battery 1 and/or causes growth of theSEI layer 114. This is believed to cause additional heating of thebattery 1 during the charging cycle which, again, has an impact on speedat which the battery 1 can be charged, the nature of the charging cyclewhich can be used, the efficiency of the charging process, the safety ofthe charging process, and/or the lifespan of the battery in terms of thenumber of charging cycles which the battery 1 can undergo before chargecapacity drops below acceptable levels.

With reference to FIG. 2, embodiments of the present invention,therefore, include a battery charging system 100 which is configured tosupply electrical current to the battery 1, also referred to as acharging current, wherein the electrical current may be of a waveform ofvarying amplitude with a generally non-zero mean average current and maybe a waveform with a substantially constant mean average current (andthat waveform may be substantially sinusoidal for example). Furtherexamples of a suitable waveform include an oscillating DC waveform or apulsed DC waveform. Such a waveform can, for example, be supplied by aswitched-mode power converter along with a controller adapted to varythe switching frequency to provide a current with a desired waveform.

FIGS. 3a-3c show the electrical current delivered to the battery 1 bythe battery charging system 100 of some embodiments (FIG. 3a ), thevoltage of the mains power supply 102 (FIG. 3b ), and the current drawnfrom the electrical power supply (FIG. 3c ), for a mains power supply102 with an RMS voltage of about 85V. FIGS. 3d-4f show the electricalcurrent delivered to the battery 1 by the battery charging system 100 ofsome embodiments (FIG. 3d ), the voltage of the mains power supply 102(FIG. 3e ), and the current drawn from the electrical power supply (FIG.3f ), for a mains power supply 102 with an RMS voltage of about 265V.

The frequency of the electrical current supplied to the battery 1 by thebattery charging system 100 may be generally about twice the frequencyof an electrical power supply to the battery charging system 100 and/orthe electrical current supplied to the battery 1 by the battery chargingsystem 100 may have a waveform substantially corresponding to arectified form of a waveform of a supply voltage (e.g. a full-waverectified form). The frequency of the electrical current supplied to thebattery 1 may be, for example, 100 Hz or 120 Hz. The magnitude of thecurrent supplied to the battery 1 may be generally equal to the peakcurrent supplied to the battery 1.

Accordingly, in embodiments of the present invention, the batterycharging system 100 may be configured to supply electrical power to thebattery 1 during a charging cycle in a first operation as describedabove. The battery charging system 100 may be configured to supplyelectrical power to the battery 1 during the same charging cycle in asecond operation in which, for example, the electrical power is suppliedwith a substantially constant voltage or even a constant power.

As can be seen from the examples in FIGS. 3a-3c and 4a-4c , theelectrical power supply voltage (V_(ac)) may be typical of the mains ACelectrical supply voltage which is available in many countries—thevoltage and frequency of which may be different in different countries(e.g. 85-265V RMS, and 50-60 Hz). V_(ac) is, therefore, typically ofsinusoidal form centred on 0V. The electrical current (I_(bat)) suppliedto the battery 1 may have a waveform which has a frequency which isrelated to that of V_(ac) and which may be, for example, substantiallytwice the frequency of V_(ac), or about an integer multiple of thefrequency of V_(ac). The electrical current (I_(bat)) supplied to thebattery 1 may have a non-zero mean average (i.e. may have an offset from0 A) and may be of a generally sinusoidal form—as described herein. Theelectrical current (I_(bat)) may be generally of the form of a full-waverectified waveform of the supply voltage V_(ac) and so, in someembodiments, the lower peaks of the waveform may be sharper thandepicted in FIGS. 4a and 4b , for example.

The waveform of the electrical current, I_(bat), supplied to the battery1, may vary between a low current (i.e. the lowest current of thewaveform) and a peak current (i.e. the highest current of the waveform).The low current may be substantially 0 A. The peak current may bebetween about 1 A and about 30 A, or between about 10 A and about 30 A,or between about 20 A and about 30 A. The peak-to-peak variation in theelectrical current, I_(bat), supplied to the battery 1 (i.e. thedifference between the peak current and the low current) may be greaterthan about 1 A, or greater than about 10 A, or greater than about 20 Aor about 30 A. Generally this peak battery charging current is about 10times the capacity of the battery. For example, the battery we use is2.6 Ah. Thus the peak current is about 26 Amps, giving an averagebattery charging current of about 16 Amps.

The waveform of the electrical current, I_(bat), may repetitivelyoscillate between the low current and the peak current. In someembodiments, this waveform is a substantially smooth waveform. In someembodiments, the waveform is substantially a full-wave rectifiedsinusoidal shape. In some embodiments the waveform partially has theform of a rectified sine wave and partially has the form of a sinesquared wave. For example, the portion of the waveform around thewaveform maximum can have substantially the form of a rectified sinewave, while the portion of the waveform around the waveform minimum canhave substantially the form of a sine squared wave (also herein referredto as a squared sine wave).

It is believed that the provision of the non-zero mean electricalcurrent waveform (I_(bat)) to the battery 1 during charging, helps toreduce the pile-up of ions as described above. This, in turn, reducesthe electrical resistance of the battery 1 during the charging cycle.This improves the efficiency of the charging of the battery 1, reducesthe heat generated during the charging process and enables safercharging of the battery 1, with a higher overall charging current. Inaddition, the first operation of the charging cycle can be used untilthe battery 1 is at a greater relative level of charge (e.g. 80% ormore, or 90% or more of the total charge capacity, or between 90% and95% of the total charge capacity) than with some conventional chargecycles.

Indeed, in some embodiments, the second operation of the charge cyclemay be omitted entirely.

Some embodiments may include a third operation of the charge cycle inwhich electrical power is occasionally (e.g. periodically) supplied tothe battery 1 to compensate for self-discharge of the battery 1.

As will be appreciated, the battery charging system 100 may beimplemented in a number of different manners. However, in someembodiments, additional advantages can be achieved by implementing thebattery charging system 100 in accordance with particular embodimentsdescribed herein—as will become apparent.

With reference to FIGS. 4-7 embodiments of the battery charging system100 are discussed in more detail.

Some embodiments of the battery charging system 100 may include aflyback transformer (also referred to herein as a flyback converter)101. The flyback transformer 101 may be configured to be connected inelectrical communication with a mains power supply 102. The mains powersupply 102 may be an AC mains power supply delivering electrical powerat the supply voltage V_(ac). The battery charging system 100 mayinclude a rectifier circuit 103 which is configured to receiveelectrical power from the mains power supply 102 and to rectify theelectrical power from the mains power supply 102 for delivery to theflyback transformer 101.

An electromagnetic interference filter 104 of the battery chargingsystem 100 may be connected in electrical communication between themains power supply 102 and flyback transformer 101. In some embodiments,the electromagnetic interference filter 104 and the rectifier circuit103 may be combined into a single circuit.

The battery charging system 100 may further include a snubber circuit105. The snubber circuit 105 may be connected between the mains powersupply 102 and the flyback transformer 101, and in particular may beconnected between the electromagnetic interference filter 104 and/or therectifier circuit 103 and the flyback transformer 101. The snubbercircuit 105 may be configured to attenuate voltage spikes which mightotherwise damage a primary switch device 101 a of the flybacktransformer 101 and/or may be configured to re-circulate current fromthe flyback transformer 101.

The primary switch device 101 a of the flyback transformer 101 isconfigured to control the delivery of electrical power from the mainspower supply 102 (and, for example, the rectifier circuit 103 and/orelectromagnetic filter 104 and/or snubber circuit 105) to a firstprimary winding 101 b of the flyback transformer 101.

The flyback transformer 101 further includes a first secondary winding101 c—the flow of electrical current through the first primary winding101 b being configured to induce the flow of an electrical current inthe first secondary winding 101 c.

The first secondary winding 101 c is connected in electricalcommunication with an output rectifier circuit 106 which is configuredto rectify the electrical power output from the flyback transformer 101(i.e. the electrical current induced in the first secondary winding 101c).

The battery charging system 100 includes terminals 107 which areconfigured to be connected to the terminals 12,13 of the battery 1 (insome embodiments, the terminals 107 of the battery charging system 100may be configured for selective connection to the terminals 12,13 of thebattery 1). The terminals 107 of the battery charging system 100 areconnected in electrical communication with the first secondary winding101 c. The connection is via the output rectifier circuit 106 such thatthe output rectifier circuit 106 is configured to rectify the electricalpower output by the first secondary winding 101 c for delivery to thebattery 1 (which is connected to the terminals 107 of the batterycharging system 100).

In some embodiments, the output rectifier circuit 106 is a synchronousrectifier circuit which includes one or more switch devices—such as atransistor device (e.g. a MOSFET).

In some embodiments, the battery charging system 100 may include asmoothing circuit 108 which is connected in electrical communicationbetween the first secondary winding 101 c and the terminals 107 of thebattery charging system 100. The smoothing circuit 108 is configured tofilter and/or smooth the current and/or voltage of the electrical powerwhich is delivered to the terminals 107 (and so on to the battery 1) ofthe battery charging system 100.

In some embodiments, the smoothing circuit 108 may include one or morecapacitors and/or inductors. In some embodiments, one or more capacitorsare provided and are connected in parallel between the terminals 107 ofthe battery charging system 100, such that the capacitors form a bank ofcapacitors. In some embodiments, which may or may not include a bank ofcapacitors as part of the smoothing circuit 108, the smoothing circuit108 includes at least one inductor which is connected in series betweenthe first secondary winding 101 c and one of the terminals 107 of thebattery charging system 100.

The one or more capacitors of the smoothing circuit 108 may include oneor more polymer electrolytic capacitors and may have a low internalresistance along with being capable of high ripple current.

The battery charging system 100 may further include, in someembodiments, an isolation switch 109 which is configured to disconnectone or both of the terminals 107 of the battery charging system 100 fromelectrical communication with at least the first secondary winding 101 cbut may also disconnect the or each terminal 107 from electricalcommunication with the output rectifier circuit 106 and/or the smoothingcircuit 108 (or a part thereof).

The battery charging system 100 may further include a control sub-system110. The control sub-system 110 is configured to control the operationof the battery charging system 100 including, for example, one or moreof the flyback transformer 101 (e.g. the primary switch device 101 athereof), the rectifier circuit 103, the output rectifier circuit 106,and the isolation switch 109.

The control sub-system 110 may further include one or more sensorcircuits and/or elements which are each configured to measure and/ordetermine one or more properties associated with the operation of thebattery charging system 100 and/or the battery 1 such that the controlsub-system 110 can control the operation of the battery charging system100 based, at least in part, on the information received from the one ormore sensor circuits and/or elements. For example, properties of thecharging current can be varied in dependence on the determinedproperties, such as the battery impedance for example, that aredetermined substantially continuously during charging.

The one or more sensor circuits and/or elements may include anelectrical supply voltage sensor circuit 1111 which is configured tosense an electrical voltage of the mains electrical supply 102—which maybe the voltage after rectification by the rectifier circuit 103, ifprovided. The electrical supply voltage sensor circuit 1111 may includea potential divider circuit which is configured to reduce a voltage to alevel which can be used by other parts of the control sub-system 110.

The one or more sensor circuits and/or elements may include a primarywinding current sensor circuit 1112 which is configured to sense anelectrical current delivered to the first primary winding 101 b of theflyback transformer 101. The primary winding current sensor circuit 1112may include a shunt circuit, for example. The primary winding currentsensor circuit 1112 may be configured to sense an electrical currentpassing through the primary switch device 101 a from the first primarywinding 101 b to ground or to the mains power supply 102.

The one or more sensor circuits and/or elements may include a batteryterminal voltage sensor circuit 1113 which is configured to sense anelectrical voltage, V_(bat), across the terminals 107 of the batterycharging system 100 and, hence, across the terminals 12,13 of thebattery 1 connected thereto. The battery terminal voltage sensor circuit1113 may include a potential divider circuit which is configured toreduce the voltage to a level which can be used by other parts of thecontrol sub-system 110.

The one or more sensor circuits and/or elements may include a batteryterminal current sensor circuit 1114 which is configured to sense anelectrical current delivered to or returned from at least one of theterminals 107 of the battery charging system 100 and, hence, deliveredto the battery 1 connected thereto. The battery terminal current sensorcircuit 1114 may include a shunt circuit, for example. The batteryterminal current sensor circuit 1114 may be configured to determine anelectrical current passing through the output rectifier circuit 106 froma terminal 107 of the battery charging system 100 to the first secondarywinding 101 c of the flyback transformer 101.

The control sub-system 110 may, in some embodiments, include a primaryor input side controller 1115 and a secondary or output side controller1116. The primary side controller 1115 may be configured to communicatewith the output side controller 1116, and/or vice versa, through acommunication channel which may include an opto-coupler circuit 1117.The opto-coupler circuit 1117 may be configured to allow communicationbetween the primary and secondary side controllers 1115,1116 whilstkeeping the two controllers 1115,1116 electrically isolated. As will beappreciated, therefore, the opto-coupler circuit 1117 may include alight emitting diode controlled by the primary side controller 1115 anda phototransistor connected to the secondary side controller 1116.

In some embodiments, a second primary or first “bias” winding 101 d isprovided as part of the flyback transformer 101. The second primarywinding 101 d may be configured, in association with the first primarywinding 101 b, such that a current passing through the first primarywinding 101 b will also induce a current in the second primary winding101 d. The electrical power, therefore, induced in the second primarywinding 101 d may be used to provide electrical power to one or more ofthe parts of the control sub-system 110—e.g. to the primary sidecontroller 1115. A first bias circuit 101 e may be provided to ensurethe correct delivery of electrical power to the primary side controller1115.

The voltage of the output from the second primary winding 101 d isreferred to herein as the “primary bias voltage”, V_(bias-primary).Besides, the second primary, or first bias winding is used to detect themagnetic state of the flyback transformer, thus assuring the criticalconduction mode of operation of the flyback converter.

In some embodiments, the flyback transformer 101 may include a secondsecondary winding 101 g or second “bias” winding. The second secondarywinding 101 g may be configured, in association with the first primarywinding 101 b, such that a current passing through the first primarywinding 101 b will also induce a current in the second secondary winding101 g. The electrical power, therefore, induced in the second secondarywinding 101 g may be used to provide electrical power to one or more ofthe parts of the control sub-system 110—e.g. to the secondary sidecontroller 1116. A second bias circuit 101 f may be provided to ensurethe correct delivery of electrical power to the secondary sidecontroller 1116. The voltage of the output from the second secondarywinding 101 g is referred to herein as the “secondary bias voltage”,V_(bias-secondary).

In the depicted embodiment, the first and second bias circuits 101 e,fare depicted in multiple parts for simplicity of the representation. Itwill be appreciated, however, that they may be respective singlecircuits or connections.

The secondary side controller 1116 is configured to compare a signalrepresentative of the voltage of across the terminals 107 of the batterycharging system 100 (e.g. the output from the battery terminal voltagesensor circuit 1113) with a battery terminal reference voltage.

The secondary side controller 1116 may be further configured to comparea signal representative of the current delivered to or returned from atleast one of the terminals 107 of the battery charging system 100 (e.g.the output from the battery terminal current sensor circuit 1114) with abattery reference current.

The secondary side controller 1116 may be configured to use the resultsof the voltage and/or current comparisons in order to determine a modeof operation for the battery charging system 100. This mode of operationmay include determining which of the first, second, or third operationof the charging cycle to use. In some embodiments multiple chargingmodes and/or duty cycles are available.

In some embodiments, the secondary side controller 1116 may beconfigured to receive a signal indicative of the temperature of thebattery 1—e.g. from a battery temperature sensor 1118 configured tosense a temperature of the battery 1 or a parameter representative ofthat temperature and to output a signal accordingly. The secondary sidecontroller 1116 of these embodiments may use the signal indicative ofthe temperature of the battery 1 to determine which of the multipleoperations of the charging cycle to use (in addition to or as analternative to using the comparison or comparisons discussed above).

The secondary side controller 1116 is, therefore, configured to output acontrol signal, C. This control signal, C, may be communicated to theprimary side controller 1115 (e.g. via the opto-coupler circuit 1117)and the primary side controller 1115 may use this control signal, C, tocontrol the operation of the flyback transformer 101—as describedherein.

The secondary side controller 1116 may be further configured to operatethe isolation switch 109 if one or more predetermined conditions occurand these may include one or more of a particular current delivered toor received from the battery 1 exceeding or falling below apredetermined threshold current, and/or a voltage across the terminals12,13 of the battery 1 exceeding or falling below a predeterminedthreshold voltage, and/or the battery temperature falling below orexceeding a predetermined threshold temperature.

The secondary side controller 1116 may be configured to control theoperation of the output rectifier circuit 106 (which may be asynchronous rectifier circuit) based on the output of a signalindicative of the electrical current delivered to the terminals 107 ofthe battery charging system 100 (i.e. to the battery 1 connectedthereto) and/or based on a signal indicative of the current passingthrough the output rectifier circuit 106. This signal may, as will beappreciated, be the output from the battery terminal current sensorcircuit 1114. In some embodiments, the secondary side controller 1116 isconfigured to actuate the output rectifier circuit 106 to an on-statewhen the flyback transformer 101 is discharging to the terminals 107 ofthe battery charging system 100 and to disable the output rectifiercircuit 106 to an off-state when the flyback transformer 101 is notdischarging to the terminals 107.

The primary side controller 1115 is configured to control the operationof the flyback transformer 101 and, in particular, to control theactuation of the primary switch device 101 a. The primary sidecontroller 1115 is configured to control the operation of the flybacktransformer 101 based at least in part on one or more of the controlsignal, C, a signal representative of the electrical current deliveredto the first primary winding 101 b (e.g. the output of the primarywinding current sensor circuit 1112), a signal representative of theelectrical voltage (V_(ac)) of the mains power supply 102 afterrectification by the rectifier circuit 103 (e.g. the output of theelectrical supply voltage sensor circuit 1111), and the AC bias voltage,V_(bias) before the bias rectification circuit.

The primary side controller 1115 is configured to operate the flybacktransformer 101 in a critical conduction mode of operation.

Accordingly, the primary side controller 1115 may be configured toactuate the primary switch device 101 a to an on-state (such thatelectrical power may pass from the mains power supply 102 to the firstprimary winding 101 b) once it has been determined that the energystored in the flyback transformer 101 (e.g. in the windings 101 b,101c,101 d, and any core thereof) has been dissipated—i.e. delivered to theterminals 107 of the battery charging system 100 and to the battery 1(or otherwise lost).

With the primary switch device 101 a in its on-state, electrical poweris delivered to the first primary winding 101 b such that there is abuild-up of energy stored in the flyback transformer 101 which, in turn,is delivered via the first secondary winding 101 c as electrical powerto the terminals 107 of the battery charging system 100 and to anybattery 1 connected thereto.

The primary side controller 1115 may be further configured to actuatethe primary switch device 101 a to an off-state (such that electricalpower is substantially prevented or hindered from passing from the mainspower supply 102 to the first primary winding 101 b) when the signalrepresentative of the electrical current delivered to the first primarywinding 101 b has reached a value which is dependent on the signalrepresentative of the electrical voltage of the mains power supply 102,and the control signal, C.

The primary side controller 1115 may be configured, therefore, toactuate the primary switch device 101 a to have a switching frequency(i.e. a frequency of switching between is on- and off-states) whichvaries over time.

The primary side controller 1115 may actuate the primary switch device101 a such that the electrical current flowing through the first primarywinding 101 b has an envelope which may generally follow the voltage ofthe mains power supply 102 (or the rectified mains power supplyvoltage). Within the envelope, the current flowing through the firstprimary winding 101 b may be pulsed with a varying frequency and/or dutycycle (the frequency and/or duty cycle determining the magnitude of thecurrent) and the frequency and/or duty cycle being controlled by theactuation of the primary switch device 101 a. FIG. 8 shows a graphicalrepresentation of the current delivered to the first primary winding 101b and shows the envelope of this current. FIGS. 9 and 10 show the pulsesof current which form part of that envelope at different times.

Accordingly, during an initial period, the frequency of actuation of theprimary switch device 101 a (and hence the current in the first primarywinding 101 b) may be relatively high (see FIG. 9, for example) (and/orthe duty cycle may be relatively low), with the frequency of theactuation decreasing (and/or the duty cycle increasing) as the magnitudeof the envelope of the current increases (see FIG. 10, for example),before the frequency of actuation increases again (and/or the duty cycledecreases) as the magnitude of the envelope of the current decreases(see FIG. 9 again, for example). Accordingly, the frequency and/or dutycycle of actuation of the primary switch device varies over time and thefrequency and/or duty cycle at any given time is dependent on themagnitude of the voltage of the electrical supply, V_(ac), at that time.

This operation of the primary side controller 1115 and primary switchdevice 101 a helps to ensure an improved power factor (which may be 0.9or above).

In some embodiments, the primary side controller 1115 may be implementedusing a circuit such as depicted in FIG. 7. In this embodiment, theprimary side controller 1115 includes a variable gain amplifier 1115 awhich is configured to receive the signal representative of theelectrical voltage of the mains power supply 102 and to attenuate oramplify that signal using the control signal, C. The output from thisvariable gain amplifier 1115 a is passed to a first comparator 1115 b ofthe primary side controller 1115 (e.g. to a non-inverting inputthereof). The signal representative of the electrical current deliveredto the first primary winding 101 b may be passed to an inverting inputof the first comparator 1115 b. The output of the first comparator 1115b may be connected to a reset input of a set-reset flip-flop 1115 c ofthe primary side controller 1115. The bias voltage (or a signalrepresentative of the bias voltage) may be connected to an invertinginput of a second comparator 1115 d of the primary side controller 1115.A non-inverting input of the second comparator 1115 d may be connectedto ground. An output of the second comparator 1115 d may be connected toa set input of the set-reset flip-flop 1115 c, and an output of theset-reset flip-flop 1115 c may be connected to the primary switch device101 a to control its actuation.

In some embodiments, the primary switch device 101 a is a transistordevice such as a Power MOSFET and, in such embodiments, the output ofthe set-reset-flip-flop 1115 c may be connected to a gate of the MOSFETforming the primary switch device 101 a.

In some embodiments, the flyback transformer 101 is of a gapped ferritecore flyback type which may have four interleaved windings—to reduceleakage inductance. The flyback transformer 101 may, for example, be ahigh frequency transformer 101 configured to operate at up to 150 W percore.

The bias circuit first 101 e may include one or more fast rectifierdiodes in order to provide electrical power to the primary controllersub-system 110.

The frequency of operation of the flyback transformer 101—e.g. theswitching frequency of the primary switch device 101 a—may be variedbetween about 40 KHz and 200 KHz.

As will be appreciated, many conventional battery charging systems 100use a Schottky rectifier diode in the output rectifier circuit 106. Insome embodiments, of the present invention, however, the use of asynchronous rectifier removes the need for this Schottky rectifierdiode—which can reduce cost, improve efficiency, eliminate the need of aheatsink and reduce the physical space required for the circuit.

The controller sub-system 110 may be configured such that the electriccurrent delivered to the terminals 107 of the average battery chargingsystem 100 (and so to the battery 1) is around 16 A. The controllersub-system 110 may be configured such that the electric currentdelivered to the terminals 107 of the battery charging system 100 (andso to the battery 1) varies between 0 A and 30 A.

The controller sub-system 110 may be configured such that the isolationswitch 109 is actuated to its off-state when the signal indicative ofthe voltage across the terminals 107 of the battery charging system 100reaches or exceeds about 4V and/or when the battery 1 is disconnectedfrom the terminals 107.

In some embodiments, the secondary side controller 1116 acts as a mastercontroller which is configured to control the operation of the primaryside controller 1115—which may, therefore, be referred to as a slavecontroller. The secondary side controller 1116 may, in such embodiments,be configured to communicate with one or more systems outside of thebattery charging system 100 to control one or more aspects of thebattery charging system's 100 operation.

The secondary side controller 1116 (and primary controller 1116) can beconfigured to vary properties of the charging current in dependence ondetermined properties of the battery during charging. Examples ofdetermined properties of the battery and properties of the chargingcurrent are given below

In some embodiments, the control sub-system 110 is configured to monitorthe electrical impedance of the battery 1. The delivery of the chargingcurrent to the battery 1 by the charging system 100 to charge thebattery 1 may then be based, at least in part, on the electricalimpedance of the battery 1. Properties of the charging current cantherefore be dependent on the determined electrical impedance of thebattery 1.

As discussed above, in some embodiments, the secondary side controller1116 may be configured to determine a mode of operation for the batterycharging system 100. Accordingly, it may be the secondary sidecontroller 1116 which is configured to monitor the electrical impedanceof the battery 1 and to use this information to control an aspect of thedelivery of electrical current to the battery 1 by the charging system100.

More specifically, in some embodiments, the secondary side controller1116 is configured to receive a signal representative of the currentdelivered to or returned from at least one of the terminals 107 of thebattery charging system 100 (e.g. the output from the battery terminalcurrent sensor circuit 1114), I_(bat). Alternatively or additionally,the secondary side controller 1116 is configured to receive a signalrepresentative of the change in current delivered to or returned from atleast one of the terminals 107 of the battery charging system 100 duringa portion of the charging current waveform delivery. An example of sucha change in current may be the difference between the peak (maximum) andtrough (minimum) charging current. In embodiments where the minimumcurrent is zero amps, this difference is equal to the maximum currentmeasured.

The delivery of electrical current, I_(bat), to the battery 1 connectedto the terminals 107 according to the current waveforms described hereincauses the voltage across the terminals 12,13 of the battery 1 to rippleas a result of the battery's electrical impedance, Z_(bat). This can beseen in FIGS. 11a and 11b , in which FIG. 11a shows I_(bat) and FIG. 11bshows the voltage, V_(bat), across the terminals 12,13 of the battery 1in an example use of some embodiments of the invention.

The secondary side controller 1116 may be configured, therefore, to usethe signal representative of the voltage of across the terminals 107 ofthe battery charging system 100 (e.g. the output from the batteryterminal voltage sensor circuit 1113) to determine the electricalimpedance of the battery 1, Z_(bat), connected to those terminals 107(by its terminals 12,13).

More specifically, the secondary side controller 1116 may be configuredto receive the signal representative of the voltage across the terminals107 of the battery charging system 100 over a first period of time. Thesecondary side controller 1116 may be configured to determine a DCcomponent of that signal (and hence the voltage it represents). This maybe achieved by, for example, determining a mean average of that signalover the first period and taking this to be the DC component. This DCcomponent is then subtracted from the signal to generate a voltageripple signal representative of the ripple voltage of across theterminals 107 of the battery charging system 100 (and hence across theterminals 12,13 of the connected battery 1), ΔV_(bat). This signal maybe amplified and/or filtered in order to improve the signal'srepresentation of the ripple voltage. For example, the frequency of theripple voltage is expected to be substantially identical to that of thecurrent delivered to or returned from at least one of the terminals 107;therefore, one or more filters may be used to attenuate frequencycomponents of the ripple voltage signal which are above and/or belowthis frequency. An example of the voltage ripple signal can be seen inin FIG. 11c , for example.

The secondary side controller 1116 may be configured to divide thesignal representative of the ripple voltage, ΔV_(bat), by a signalrepresentative of a change in current delivered to or returned from atleast one of the terminals 107 of the battery charging system 100,ΔI_(bat), in order to arrive at the electrical impedance of the battery1 connected to those terminals 107.

In some embodiments, the current delivered to or returned from at leastone of the terminals 107 of the battery charging system 100, I_(bat),may vary from 0 A to a peak (maximum) current (before returning to 0 Afollowing the waveforms described herein). Therefore, the signalrepresentative of I_(bat) may be substantially equal to the signalrepresentative of ΔI_(bat) and may be used as such. However, in someembodiments, I_(bat), varies between a non-zero lower value and a peakcurrent. Therefore, in some embodiments, ΔI_(bat) is not equal toI_(bat). In such embodiments, the secondary side controller 1116 may beconfigured to determine ΔI_(bat) from the signal representative of thecurrent delivered to or received from the terminals 107, I_(bat).

FIGS. 11a and b illustrate the current and the voltage at the batteryterminals as charging progresses. As the battery is undergoing charging,the baseline voltage steadily increases with an overlaid ripple causedby the oscillating input current. In order to determine the impedance ofthe battery at a given point in time, only the ripple in the voltage isrelevant, and is evaluated separately from the increasing voltagebaseline. FIG. 11c shows an illustration of the voltage ripple. In orderto determine the magnitude of the ripple, for example a linear baselinevoltage is fitted to the voltage curve and the deviation from thatbaseline is evaluated. That deviation gives the flattened ripple. Theflattened ripple can then be evaluated to determine the effect of theimpedance on the voltage. The magnitude of the ripple voltage divided bythe magnitude of the battery current gives the magnitude of theimpedance. In another example a mean average of the voltage signal in aperiod is determined and subtracted from the voltage signal; this againgives the ripple as distinct from the baseline voltage. In the case ofnon-oscillating steady charging the battery voltage increases steadily,and no signal is available that is indicative of the impedance of thebattery.

The internal impedance of the battery and the battery internal batteryvoltage are not generally accessible for direct measurement while thebattery is being charged. By dividing the change in current by thechange in voltage the battery internal impedance of the battery can bedetermined without the need to know the internal battery voltage.

The secondary side controller 1116 may be configured to determine theelectrical impedance of the battery 1 using the above processperiodically. In some embodiments, the secondary side controller 1116 isconfigured to determine the electrical impedance of the battery 1 at afrequency which is substantially equal to the frequency of the signalrepresentative of the current delivered to or received from theterminals 107 of the battery charging system 100. In other words, thesecondary side controller 1116 may be configured to determine theelectrical impedance of the battery 1 once for every peak in the signalrepresentative of the current delivered to or received from theterminals 107 of the battery charging system 100, thereby effectivelydetermining the electrical impedance of the battery 1 substantiallycontinuously.

In some embodiments, the secondary side controller 1116 may beconfigured to determine the electrical impedance of the battery 1 at afrequency which is substantially equal to at least the frequency of thesignal representative of the current delivered to or received from theterminals 107 of the battery charging system 100, thereby effectivelydetermining the electrical impedance of the battery 1 substantiallycontinuously. In such embodiments, the secondary side controller 1116may be configured to determine the impedance of the battery 1 using therising or the falling peak-to-peak variation in the signalrepresentative of I_(bat).

In some embodiments, the secondary side controller 1116 is configured todetermine the impedance of the battery 1 at a frequency or between about100 Hz and 120 Hz, or at a frequency of about 120 Hz, or between 100 Hzand 200 Hz, or at a frequency of about 200 Hz, or between 120 Hz and 240Hz, or at about 240 Hz, or at any combination of the upper and lowerbounds of these ranges.

As will be appreciated, in some embodiments, whilst the determining ofthe impedance of the battery 1 is periodic in absolute terms, thedetermining is substantially continuous relative to the frequency atwhich the secondary side controller 1115 may be configured to causechanges in I_(bat) during a charging cycle—see below. It will beappreciated that this will still effectively be the case if theimpedance of the battery is not determined at exactly the frequency ofthe charging current waveform, but is instead determined at a frequencythat is substantially continuous relative to the frequency at which thesecondary side controller 1115 may be configured to cause changes inI_(bat) during a charging cycle.

The change in current and change in voltage supplied to the battery 1can be measured during the rising current portion of the currentwaveform. The minimum of the current waveform can provide a convenientstarting point to measure these properties from. The changes canalternatively or additionally be measured on the falling current portionof the current waveform. When these properties are measured on both therising and falling portion of the waveform, the results can be comparedby the secondary side controller 1115 in order to determine if there isan error.

The control sub-system 110 may include a look-up table 1119. Thislook-up table 1119 may be stored on a computer readable medium 1119 a ofthe control sub-system 110 or to which the control sub-system 110 hasaccess. The look-up table 1119 may, in some embodiments, be part of oraccessible by the secondary side controller 1116.

The look-up table 1119 may store information which correlates batterycondition and/or charge state to charging mode. Accordingly, the look-uptable 1119 may be used by the control sub-system 110 (e.g. the secondaryside controller 1116) to determine an appropriate charging mode for thebattery 1 based on determined properties about the battery 1 which isavailable to the control sub-system 110.

These determined properties available to the control sub-system 110(e.g. to the secondary side controller 1116) may include one or more of:I_(bat), V_(bat), ΔI_(bat), ΔV_(bat), and Z_(bat). This information mayalso include the temperature of the battery T_(bat) 1. In embodimentswhere the battery comprises multiple cells, these properties for eachcell may be available to the control subsystem in addition to oralternatively to the complete battery properties.

In some embodiments, the information available to the control sub-system110 (e.g. the secondary side controller 1116) may include an identity ofthe battery 1—which may be determined, for example, by a batteryidentifier which is communicated to the control sub-system 110. Thebattery identifier may be an identifier which is unique or substantiallyunique to that battery 1 (such as a serial number) or may be more a moregeneral identifier which is unique or substantially unique to thatmanufacturer of battery 1 or that model of battery 1 or thatconfiguration of battery 1, for example. The identity may becommunicated to the control sub-system 110 (e.g. the secondary sidecontroller 116) by one or more signals transmitted from the battery 1 tothe control sub-system 110 (e.g. via a communication link between thetwo) or may be manually entered by a user.

The control subsystem 110 may further comprises a time measurementmeans, such as a clock. This can be used to measure the time the battery1 has been charging. By integrating the time the battery 1 has beencharging with respect to the charging current, the total chargedelivered to the battery can be determined. This integration can, forexample, be performed numerically. The total charge can be used todetermine the State of Charge of the battery 1.

The control sub-system 110 (e.g. the secondary side controller 1116) mayuse the determined properties available to it (see above) to determinethe current charge state and/or health of the battery 1. As will beappreciated, the health of a battery 1 is a measure of the correctoperation of the battery 1 (such as its ability to hold charge). Thecurrent charge state of the battery 1 may be otherwise referred to asthe current condition of the battery 1 or the battery state of charge(S.O.C.). These can be used to determine if and how the charging currentsupplied to the battery 1 should be varied.

In some embodiments, the battery impedance is not determined, andinstead the change in current ΔI_(bat) and change in voltage ΔV_(bat)can be used directly to determine the current charge state and/or healthof the battery, and therefore determine if and how the charging currentshould be varied.

In some embodiments, the look-up table 1119 provides a duty cycle and/orfrequency for the operation of the primary switch device 101 a based onthe determined properties available to the control sub-system 110 (e.g.to the secondary side controller 1116)—see above. In some embodiments,the look-up table 1119 is used to provide an indication of a desiredI_(bat) for the condition and/or health of the battery 1. The look-uptable 1119 may, for example, provide C (see above). Other properties ofthe battery charging current, such as: a current mean value; a currentmaximum value; a current amplitude; a duty cycle; and/or a charging modemay also be provided in the look up table.

For example, a fully discharged battery 1 will have a relatively highelectrical impedance and so the look-up table 1119 may specify arelatively low I_(bat) in order to avoid the risk of overheating thebattery 1 and/or otherwise damaging the battery 1. Likewise, if there isa low determined impedance, then the look-up table 1119 may specify arelatively high I_(bat).

The above description of the operation of the control sub-system 110refers to the monitoring of the electrical impedance of a battery 1. Asis mentioned above, this is to be construed as monitoring the electricalimpedance of one or more cells 11 of that battery 1.

In some embodiments, a battery 1 includes a plurality of cells 11. Insuch embodiments, the impedance of each cell 11 may be determined—theabove operations being performed on each cell 11 in the battery 1—andthe charging mode may be determined based on the information determinedfor more than one cell 11 of the battery 1.

Embodiments of the present invention have been described with referenceto a flyback converter 101, but it will be appreciated that these andother embodiments may be implemented by using a converter with adifferent topology. Such a converter may be capable of high frequencyoperation (e.g. operation in the range of 40 to 200 kHz).

A single battery charging system 100 has been described herein. However,it will be understood, that a plurality of such battery charging systems100 may be provided connected in parallel with each other and configuredto supply an electrical current (I_(bat)) to the same terminals 107. Insuch an arrangement, all of the battery charging systems 100 may becontrolled by a common control sub-system 110. In some embodiments, sucharrangements may include a primary side controller 1115 for each batterycharging system 100 but a single secondary side controller 1116 which isconfigured to control the operation of all of the primary sidecontrollers 1115.

As will be appreciated, embodiments supply the electrical currentI_(bat) having a waveform which allows the electrical impedance of thebattery 1 to be determined frequently without the need for anyadditional manipulation of I_(bat) for the purpose of determining thebattery's electrical impedance. In other words, the electrical currentwhich is used to charge the battery and which forms the main current tocharge the battery 1 (i.e. a charging current) may be used to determinethe battery's electrical impedance. This is different, for example, froma relatively, small magnitude, current which is deliberately added to aconstant or substantially constant main charging current in order todetermine the battery's electrical impedance. In such a hypotheticalarrangement, the maximum frequency at which the electrical impedance ofthe battery 1 can be determined is much lower than in some embodimentsof the present invention. In addition, the small magnitude of thecurrent does not contribute substantively to the charging of the battery1 (e.g. being only of the hundreds of milliamp-level). Such a lowmagnitude current does not, therefore, form part of a main chargingcurrent—unlike the current, I_(bat), in some embodiments of the presentinvention in which the changes in the current are of a magnitude thatdoes impact the charging of the battery 1 and, therefore, forms part ofthe main charging current.

In some embodiments, the control sub-system 110 is configured tocommunicate with a remote server 210. The communication may be via acommunication network—such as a local area network and/or a wide areanetwork and may include the Internet. The remote server 210 may, forexample, store the look-up table 1119 or part thereof. In someembodiments, the remote server 210 includes the computer readable medium1119 a. In some embodiments, the control sub-system 110 is configured todownload at least part of the look-up table 1119 (e.g. to a localcomputer readable medium) 1119 a.

The control sub-system 110 (e.g. the secondary side controller 1116) maybe configured to request the whole or part of the look-up table 1119from the remote server 210 and may be able to upload information about abattery 1 to the remote server 210.

The remote server 210 may be configured to maintain the look-up table1119 and this may include receiving information about a battery 1 or atype of battery 1 from the control sub-system 110 and then updating thelook-up table 1119 based on that information. This may include updatingthe look-up table in relation to that particular battery 1 and/or forthat type of battery 1 (e.g. based on the configuration of the battery1, the make and/or model of that battery 1). The remote server 210 maybe configured to collate the information received about a plurality ofbatteries 1 in order to modify the look-up table 1119 or parts thereofto optimise one or more aspects of the operation of the battery chargingsystem 100 using the look-up table 1119—this may include ensuring themaximum amount of charge is stored in a battery 1, ensuring the maximumlifespan (i.e. number of charging cycles) of a battery 1, and/orensuring the most efficient use of electrical power by the batterycharging system 100.

FIG. 12 illustrates a step-by-step procedure for charging a battery. Theprocedure is intended to provide high power efficiency and safelyminimize the battery charging time.

1. Initialization

-   -   Recognize the type of battery (Total Battery Voltage Vbat and        Capacity Cbat)    -   Look at the data base and estimate the State of Charge (S.O.C.)        of the battery    -   Look at the data base and determine the maximum permissible cell        voltage Vcell_max    -   Measurements of all the Cells Voltages Vcell_open    -   Look at the data base and determine the initial battery charging        current Ibat(av)_initial

2. Start Charging

-   -   Start the clock    -   Start charging with rectified sine current waveform and        Ibat(av)_initial

3. Measurements

-   -   Measure in each individual cell its voltage and the battery        current at the bottom of the charging current waveform    -   Store the measurements Vcell_bottom and Ibat_bottom    -   Measure in each individual cell its voltage and the battery        current at the    -   top of the charging current waveform    -   Store the measurements Vcell_top and Ibat_top

4. Calculations

-   -   Calculate the difference ΔVcell=Vcelltop−Vcellbottom    -   Calculate the difference ΔIbat=Ibat_top−Ibat_bottom    -   Calculate the impedance of each individual cell        Zcell=ΔVcell/ΔIbat

5. Full Charging

-   -   Assign the weakest cell (with the maximum impedance) Z_weak_cell    -   Look at the data base and determine the maximum safe battery        charging current Ibat_max_safe, according to the Z_weak_cell    -   Continue battery charging, continuously measuring and        calculating all the above, making full utilization of the        charger potential and safely minimize the battery charging time    -   Continuously measure each individual cell temperature Tcell.

6. Stop

-   -   Continuously look at the data base and modify the Vcell_max and        Ibat_max_safe according to the hottest cell Tcell_max    -   Stop the charging when the first cell voltage reaches Vcell_max

7. Administration

-   -   Calculate the Time-Amps integral and specify the total charge        received by the battery.    -   Display the Battery S.O.C.    -   Store information into the database

Storing the determined properties of the battery in the database, whichcan for example reside in a look-up table stored in a controller/controlunit memory, can allow the battery charging system to correlate theseproperties for future use in charging other batteries and/or chargingthe same battery again.

The determined properties can, for example, be stored along withidentification data, associating the properties with that particularbattery. When charging that particular battery in the future, theseproperties can be used to determine the battery state and/or therequired charging current properties.

Alternatively or additionally, the stored determined properties can beaggregated into a dataset from which battery property correlations canbe determined for classes and/or types of battery. These correlationscan be used when charging batteries of the same type and/or class todetermine/estimate the battery state and/or the required chargingcurrent properties.

By storing the determined properties and using them to assist in futurebattery charging events, the system can be said to be self-learning.

FIG. 13 illustrates a schematic example of a single cell battery controlunit in use. The secondary controller (Master CPU) drives a currentsource, such as a flyback converter, to charge a battery cell with acharging current (I_(cell)), for example a current with an oscillatingDC waveform such as a rectified sinusoidal waveform, of about twice theline frequency. This direct (i.e. always positive) but time varyingcurrent can have a minimum value of less than about 0.5 amps, preferablyless than about 0.1 amps, more preferably about 0 amps, with a maximumvalue of about 30 amps.

This “ripple” battery charging current creates a ripple battery cellvoltage ΔV_(cell) due to the battery internal impedance Z_(cell). Thebattery internal impedance Z_(cell can) depend on the battery cell stateof charge, battery cell temperature T_(cell) battery cell voltageV_(cell) and battery cell history, for example.

The rippled battery voltage ΔV_(cell) can be filtered and amplified, andused to transfer battery condition information to the secondarycontroller. This can be used together with I_(cell) (and/or the changein current ΔI_(cell) if the minimum of the charging current is not zero)to calculate Z_(cell). The controller uses this information, along withthe other determined properties such as the battery cell temperatureT_(cell) and battery cell voltage V_(cell) to choose an appropriatecharging mode from a look up table. The secondary controller can thenvary properties of the charging current in dependence on the batterycell situation by, for example, changing the duty cycle of the primarycontroller.

For example, when the battery is fully discharged, the battery internalimpedance can increase. The secondary controller calculates the batteryimpedance using the rippled charging current and the voltage ripple thatthis current induces. The secondary controller then determines therequired charging mode from the look up table. In order to avoidoverheating and/or damage of the battery cell, the look up tableindicates that a different charging mode is required. The secondarycontroller sends another charging mode to the primary controller with areduced duty cycle, thereby reducing the battery charging current.

Effectively, the battery cell itself “decides” at every moment which isthe best mode for charging.

FIG. 14 illustrates a schematic example of a multi-cell battery controlunit in use. In this example the battery comprises n cells arranged inparallel and/or series. The example functions in a similar way to theexample described above in relation to FIG. 13, with the exception thatthe voltage across each cell V_(cell(i)), the change in voltage acrosseach cell ΔV_(cell(i)), and the temperature of each cell T_(cell(i)) ismeasured. The current I_(cell) will be the same through each cell. Themeasurements from all cells are passed to the secondary controller,along with the current I_(cell) (and/or the change in current I_(cell)).The battery internal impedance of each cell Z_(cell(i)) can becalculated by dividing the current (or change in current) by the changein voltage across that cell ΔV_(cell(i)).

The controller uses this information to choose from a Look Up Table themost appropriate battery charging mode, and changes the duty cycle ofthe primary controller, taking into account the situation of all thebattery cells.

In some embodiments of the invention a rectified sinusoidal current isused to charge batteries, and the voltage ripple natural to thischarging method is used (along with the current) to measure the internalimpedance of the cell. This allows the internal impedance of the cell tobe substantially continuously monitored, allowing a controller tocontinuously monitor the battery health through the internal impedancecalculation and change charging modes on the fly.

In some embodiments, a primary controller is used to control the batterycharging current. The Primary Controller consists of a Programmable Chipdevice. It senses:

-   -   the Grid Input Voltage through a High Voltage Divider    -   the Flyback Primary Current through a Primary Current Shunt        connected at the Primary MOSFET Source    -   the Control Signal coming from the Primary Controller through        the Isolating Optocoupler    -   the Bias Voltage from the Flyback Transformer

The circuit is arranged and functions as follows:

1. The Grid Input Voltage after the High Voltage Divider is amplified orattenuated by the Control Signal, through a Variable Gain Amplifierinside the programmable chip.

2. The output of this amplifier is fed to the noninverting input of acomparator inside the programmable chip.

3. The voltage from the Primary Current Shunt (connected at the PrimaryMOSFET Source) is connected to the inverting input of this comparator.

4. The output from this comparator is the Reset signal to a Set resetFlipFlop inside the programmable chip.

5. The output from the Bias Winding of the Flyback Transformer isconnected to the inverting input of another comparator inside theprogrammable chip.

6. The noninverting input of this comparator is grounded

7. The output from this comparator is the Set Signal to the Flip Flop.

8. The output from the Flip Flop is connected to the Gate of the PrimaryPower MOSFET.

So: The Gate Signal to Primary Power MOSFET can:

-   -   Start when all the Energy Stored in the Flyback Transformer has        been delivered to the Battery; and    -   End when the voltage on the Primary Current Shunt (connected at        the PrimaryMOSFET Source) has reached the value of the Input        Voltage, divided (by the High Voltage Divider) and amplified or        attenuated by the Control Signal (coming from the Primary        Controller through the Isolating Optocoupler)

In this way the Grid Current can be in phase with the Grid Voltage (PFCConverter). The Control Signal is made by the Primary Controller (whichis also another programmable chip) according to the: Grid Input Voltage,Primary Peak Current, Battery Voltage, Battery Current, BatteryTemperature

In some embodiments, a multi-phase charger 300 may be provided byconnecting a plurality of battery charging systems 100 in parallelbetween a multi-phase input 301 and an output 302. Therefore, in somesuch embodiments, the multi-phase charger 300 may include a plurality ofbattery charging systems 100 (e.g. as described herein). A batterycharging system 100 may be provided for each phase of the multi-phaseinput 301. Accordingly, in a three phase system with a three phasemulti-phase input 301 there may be three battery charging systems 100provided in the multi-phase charger 300.

Each battery charging system 100 may be electrically connected between asingle phase of the multi-phase input 301 and the output 302. In someembodiments, the output 302 is common to all of the battery chargingsystems 100 of the multi-phase input 301. Accordingly, the terminals 107of each battery charging system 100 may be connected to provide thecommon output 302 (which may have common ground and a common liveterminal). In some embodiments, therefore, the combined output from thebattery charging systems 100 of the multi-phase charger 300 may providethe charging current, as described herein. The battery charging systems100 of the multi-phase charger 300 may, therefore, be controlledaccording to achieve this effect. As such the control sub-systems 110 ofeach battery charging system 100 (if provided) may be communicativelycoupled. In some embodiments, the multi-phase input 301 provides aninput voltage of between about 200 and about 800 V_(RMS). The output 301may between about 100 and about 800 V_(DC).

When used in this specification and claims, the terms “comprises” and“comprising” and variations thereof mean that the specified features,steps or integers are included. The terms are not to be interpreted toexclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

1. A method of monitoring the internal impedance of a battery duringcharging, the method comprising: providing, by a charging system, a timevarying current to the battery; measuring, by the charging system, achange in the current substantially continuously; measuring, by thecharging system, a corresponding change in a voltage at a chargingterminal coupled to the battery substantially continuously; anddetermining, by the charging system, an internal impedance of thebattery based on the change in current and the corresponding change involtage.
 2. The method according to claim 1, wherein the time varyingcurrent is provided in the form of an oscillating waveform havingconstant polarity.
 3. The method according to claim 2, wherein thewaveform has a minimum occurring at a current value of less than about0.5 amps.
 4. The method according to claim 2, wherein a frequency atwhich the internal impedance is determined is at least an oscillationfrequency of the waveform.
 5. The method according to claim 2, whereinat least one of the change in current or the change in voltage ismeasured during a rising portion of the waveform.
 6. The methodaccording to claim 5, wherein the at least one of the change in currentor the change in voltage is measured starting at a waveform minimum. 7.The method according to claim 2, wherein at least one of the change incurrent or the change in voltage is measured during a falling portion ofthe waveform. 8-53. (canceled)
 54. A battery charger for providing acharging current to a battery, the battery charger being adapted toprovide a current with an oscillating waveform that has waveform minimaare at or near 0 amps.
 55. The battery charger according to claim 54,wherein the waveform minima are at less than 0.1 amps.
 56. The batterycharger according to claim 54, wherein a peak-to-peak variation in thecurrent is greater than 1 amp.
 57. The battery charger according toclaim 54, wherein the current is at or near 0 amps for at least 0.5 msper oscillation cycle of the waveform.
 58. (canceled)
 59. The batterycharger according to claim 54, wherein a plurality of battery chargingsystems are provided, and each battery charging system is configured toreceive an input from a single phase of a multi-phase input and togenerate a common output.
 60. A method of charging a battery, the methodcomprising converting an alternating polarity output of a power sourceto a charging current; and providing the charging current to thebattery, wherein the current has an oscillating waveform, and thewaveform has waveform minima at or near 0 amps. 61-92. (canceled) 93.The method according to claim 60, wherein the waveform minima are atless than 0.1 amp.
 94. The method according to claim 60, wherein apeak-to-peak variation in the current is greater than 1 amp.
 95. Themethod according to claim 60, wherein the waveform is at or near 0 ampsfor at least 0.5 ms per oscillation cycle of the waveform.
 96. Themethod according to claim 60, wherein a plurality of battery chargingsystems are provided, and each battery charging system is configured toreceive an input from a single phase of a multi-phase input and togenerate a common output.
 97. The method of claim 2, wherein theoscillating waveform is substantially in the form of a squared sinewave.
 98. The method of claim 60, wherein the oscillating waveform issubstantially in the form of a squared sine wave.
 99. The method ofclaim 3, wherein the minimum occurs at a current value of less thanabout 0.1 amps.
 100. The method of claim 3, wherein the minimum occursat a current value of about 0 amps.
 101. The battery charger accordingto claim 54, wherein the waveform minima are at less than 0.01 amps.102. The battery charger according to claim 54, wherein the waveformminima are at less than 0.001 amps.
 103. The battery charger accordingto claim 54, wherein a peak-to-peak variation in the current is between10 amps and about 30 amps.
 104. The battery charger according to claim54, wherein a peak-to-peak variation in the current is at least 10 timesa capacity of the battery.
 105. The battery charger according to claim54, wherein the current is at or near 0 amps for at least 1 ms peroscillation cycle of the waveform.
 106. The battery charger according toclaim 54, wherein the current is at or near 0 amps for at least 10 msper oscillation cycle of the waveform.